Highway Engineering

Basic Elements of Highway Traffic Analysis 101

Example 4.8

A 12-hour traffic count carried out on the route of a proposed motorway in

April established a volume of 45 000 vehicles.

What is the AADT figure?

Solution

Use E-factor to convert 12-hour value to a 16-hour value

45 000 ¥1.15 =51 750

Use M-factor to convert 16-hour figure to AADT

For a motorway, with the traffic count taken in April, the M-factor is 364

Therefore,

Annual traffic =51 750 ¥ 364

Annual average daily traffic =51 750 ¥ 364 ∏ 365 =51 608 vehicles.

This flow falls within the range of a 3-lane dual motorway (D3M).

4.8 Concluding comments,

The British and American methods offer contrasting methodologies for sizing
a roadway. The UK system yields a recommended design range which will offer
the designer a choice of types and widths. The final choice of option is made
by considering economic factors such as user travel and accident costs within a
cost-benefit framework. Environmental effects will also form part of the evalu-
ation. The US system, on the other hand, relates design flows to the expected
level of service to be supplied by the highway. Level of service is dependent on
factors such as traffic speed and density.

4.9 References,

Department of Transport (1985) Traffic flows and carriageway width assessments. Depart-
mental Standard TD 20/85. The Stationery Office, London, UK.
Department of Transport (1996) Traffic Input to COBA.Design Manual for Roads and
Bridges, Volume 13: Economic Assessment of Roads. The Stationery Office, London,
UK.
Department of Transport (1997a) Traffic Flow Ranges for Use in the Assessment of New
Rural Road.Departmental Advice Note TA 46/97. The Stationery Office, London, UK.
Department of Transport (1997b) The COBA Manual.Design Manual for Roads and
Bridges, Volume 13: Economic Assessment of Roads. The Stationery Office, London,
UK.